U.S. Pat. No. 5,485,902 describes a hydraulic brake piston adjuster assembly adapted to automatically adjust the clearance between the brake lining and brake disc as the lining is worn. A first piston is displaceable in a housing, and a second piston is displaceable coaxially in the first piston in a second piston chamber. During braking, the first piston is displaced in the direction of the brake disc, whereby a set of disc springs is compressed fully until an abutment is reached. Thereby, the piston cap of the second piston compresses the brake stack. If the brake stack has been worn, the second piston will be displaced relative to the first piston until abutment of the piston cap, as hydraulic fluid is allowed to enter the second piston chamber through a relief valve. Thereby, the position of the piston cap is adjusted according to wear of the brake lining, and, when releasing the brake, the piston cap will be retracted a certain distance by means of the disc springs. However, it may be complicated to retract the second piston into the first piston in order to gain the space necessary for a replacement of the brake lining, as the relief valve positioned centrally within the second piston will have to be opened, and an adequate force must be applied possibly manually to the piston cap of the second piston in order to retract the second piston into the first piston.
EP 0 413 112 A1 describes a hydraulic brake with automatic slack adjustment. The rest position of a main brake piston is adjusted by means of an auxiliary piston. The auxiliary piston is advanced as wear takes place and is retained in its position entirely by means of friction rings. Furthermore, with increasing wear of a brake lining, an increasing volume of hydraulic oil is trapped behind the auxiliary piston in the rest position of the main brake piston with the only aim of withstanding sudden impacts on the main brake piston. However, the use of friction rings may not provide a sufficient accurate adjustment of the rest position of a main brake piston. Furthermore, it may be complicated to retract the main brake piston in order to replace the brake lining, as the brake may have to be completely disassembled in order to release the friction rings.